Interleukin-12 (IL-12) is a heterodimeric cytokine defined by its ability to induce the maturation of cytolytic lymphocytes and by its capacity to effectively synergize with IL-2 in the induction of cytolytic activity. IL-12 activates NK cells, facilitates the generation of specific cytolytic T lymphocytes (CTL), maintains tumor infiltrating lymphocytes (TIL) in culture, and induces the secretion of high levels of IFN-gamma (required early in the immune response) from both T cells and NK cells in vitro. While IL-2 may promote similar functions, IL-12 is effective at concentrations 10-1000 fold less than IL-2 (on a pM basis). Since IL- 12 is (apparently) secreted exclusively by professional antigen presenting cells (APC, monocytes and B cells) and IL-12 receptors are expressed selectively on activated T lymphocytes and NK cells, it appears that IL-12 is well suited to drive the expansion and maturation of the two most prevalent anti-tumor effector cells localized within tumor lesions (T cells and NK cells). These findings indicate that il-12 may be an ideal candidate for use in immunotherapy. Further, based on the available data derived from the in vivo study of other cytokines, we believe IL-12 therapy will be optimally effective when administered through gene transfer in a paracrine (local) manner. IL-12/IL-2 synergy may allow for potent combination cytokine therapy. As a basis for future IL-12 clinical trials we propose the following specific aims: 1. Establish efficient polycistronic gene transfer systems for producing transfectants secreting biologically active IL-12 and expressing T-cell costimulatory molecules (i.e. IL-2, B7). Tumor cells and fibroblasts will be transduced with novel retroviral vectors. 2. Examine the ability of paracrine secretion of IL-12, alone or in conjunction with coexpressed costimulatory molecules, to enhance anti-tumor immune responses in vivo. Engineered tumors or engineered fibroblasts admixed with tumor will be evaluated for their capacity to elicit anti-tumor immunity in vivo. 3. Examine the IL-12 induced mechanism(s) involved in the anti-tumor response. IL-12 induced T-cell protein and gene products potentially involved directly in the anti-tumor immune response will be identified. We will develop IL-12 vectors suitable for clinical trials in the Human Gene Therapy Applications Laboratory and prepare a clinical trial.